1. Field of the Invention
The present invention relates generally to an anti-lock brake control system for a motor vehicle in which driving wheels thereof are driven by a prime mover such as an internal combustion engine by way of a torque transmitting member such a driving shaft subjected to a torsion torque. More particularly, the invention is concerned with the anti-lock brake control system which is designed for controlling brake forces applied to the wheels in initial braking phase in such manner that a short stopping distance can be realized, while ensuring a high stability for the brake operation. The invention also concerns a brake force control method which can be carried out by a microcomputer or the like.
2. Description of Related Art
In general, in the anti-lock brake control system for the motor vehicle, a trend of wheels being locked is detected on the basis of the result of comparison between the wheel speed (rpm) and an estimated speed of the motor vehicle or on the basis of deceleration of the wheel. In that case, braking hydraulic pressure applied to the wheel is so regulated that magnitude of skid of the wheel relative to the road surface is maintained at a value close to a region in which friction between the wheel and the road surface assumes a peak value, with a view to shortening the stopping distance of the motor vehicle, while ensuring stability of the motor vehicle and enhancing the manipulatability or driving performance thereof. By way of example, in the conventional anti-lock brake control system known heretofore, decision is made to the effect that the wheel of the motor vehicle tends to be locked when behavior of the wheel such as a slip thereof which represents a sink of the wheel speed relative to the estimated vehicle body speed attains a predetermined threshold value, whereupon the braking hydraulic pressure applied to the wheel is lowered under control.
As is known in the art, a reaction force exerted by a road surface to the wheels of a motor vehicle is given by a product of a friction coefficient of the road surface and a load applied to the wheel. Accordingly, upon sudden or hard application of brake, the brake force applied to the wheel increases rapidly before the load is sufficiently shifted to the wheels, as a result of which tendency of the wheel being locked increases rapidly and steeply. In that case, the anti-lock brake control system issues a command for reducing the brake force because of the wheel lock tendency mentioned above. Consequently, the brake force becomes lowered notwithstanding of the hard brake application as mentioned above. Such unwanted situation is likely to occur particularly when the motor vehicle is running on a road having high friction coefficient such as an asphalt-surfaced road.
As an attempt for coping with the phenomenon mentioned above, the conditions for allowing the braking hydraulic pressure to decrease in the initial or starting phase of the anti-lock brake control (i.e., immediately after the anti-lock brake control is started) are set more severe when compared with the braking hydraulic pressure lowering conditions adopted when the anti-lock brake control system is operating, by setting the threshold value for comparison with the wheel acceleration or slip for thereby validating reduction of the brake application pressure so that the braking hydraulic pressure is more difficult to decrease in the initial braking phase. However, by setting the brake application pressure lowering condition more severe at the start of the anti-lock brake control, there may arise a possibility that the slip of the wheel relative to the road surface increases remarkably particularly when the motor vehicle is running on a road having a road surface of low friction coefficient such as a frozen road surface. Such being the circumstances, in the conventional anti-lock brake control system, there is adopted a method for searching a compromising threshold level in an effort to solve simultaneously the two problems mentioned above.
For having better understanding of the present invention, description will first be made in some detail of the technical background of the invention. For coping with the problems mentioned above, there is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 47950/1995 (JP-A-7-47950) such a control method as illustrated in FIG. 9. Referring to the figure, when a braking hydraulic pressure P is increased, a wheel speed Vw becomes lower with a wheel acceleration Gw decreasing gradually. When the wheel acceleration Gw becomes lower than a predetermined threshold value .alpha.(i.e., when Gw&lt;.alpha.) at a time point t21, the slope or rate at which the braking hydraulic pressure P is increased is modified or changed. Subsequently, as the braking hydraulic pressure increases, the wheel speed Vw becomes lowered steeply with the brake force applied to the wheel exceeding the reaction force of the road surface. As a result of this, when the slip of the wheel reaches a predetermined value .lambda. at a time point t22, the braking hydraulic pressure is decreased, whereupon the conventional anti-lock brake control is started. In this manner, by modifying or changing once the slope or rate at which the braking hydraulic pressure is increased before starting the anti-lock brake control when the motor vehicle is running on a road having a high friction coefficient, the brake force can be increased in conformance with the load shift. Thus, there can be ensured a sufficiently high brake force even in the initial or starting phase of the anti-lock brake control. On the other hand, on a road surface of a low friction coefficient, the anti-lock brake control is started with a delay for preventing the braking hydraulic pressure from becoming excessively high. Thus, a large slip of the wheel can be suppressed in the initial phase of the anti-lock brake control, whereby running stability of the motor vehicle can be assured.
In the conventional anti-lock brake control system described above, the brake force is controlled in dependence on the wheel speed and the wheel acceleration. It is however noted that such brake force control is effectuated only when such situation arises in which the brake force has increased steeply. In that case, the driving wheels operatively coupled to the internal combustion engine serving as the prime mover by means of driving shafts such as axle shafts are forced to decelerate rapidly. However, because the internal combustion engine has a large inertia, deceleration of the engine rotation takes place at a lower rate when compared with that of the driving wheel. As a result of this, the driving shaft is subjected to torsion of a large magnitude, and thus the behavior of the driving wheel does not conform with the brake force and the reaction force of the road surface under the influence of the torsion of the driving shaft.
More specifically, when the brake force applied to the driving wheel rises up steeply, the rotation speed (rpm) of the driving wheel becomes lower when compared with that of the internal combustion engine. In that case, such a driving force acts on the driving wheel which urges the rotation speed of the driving wheel to increase to thereby prevent the rotation speed of the driving wheel from the tendency of becoming lower under the influence of the inertia of the internal combustion engine. By contrast, the engine is applied with the brake force which urges the engine to lower the rotation speed thereof. Because a torsion torque acts on the driving wheel as a driving force, the rotation speed of the driving wheel is prevented from lowering significantly even when the brake force applied to the driving wheel increases to a level comparable to the reaction force of the road surface. When the brake force is further increased and the driving wheel starts to decelerate, the conditions for changing the slope or rate at which the braking hydraulic pressure is increased are satisfied. However, at that time point, there is a possibility that the brake force exceeds the reaction force of the road surface. Accordingly, when the control for reducing the brake force is started, the braking hydraulic pressure may have been increased to an excessively high level with the brake force exceeding remarkably the reaction force exerted by the road surface.